How To Calculate Solar Panels Needed

Every solar sizing starts from consumption, not from roof size. The question is how much energy you use in a day, and how much one panel will give back where you live.

Annual consumption divided by 365 gives the daily figure. A home on 10950 kWh a year uses 30 kWh a day, and that single number drives the whole calculation.

Peak sun hours is not the length of the day. It is the equivalent number of hours at full strength, so a long cloudy day might contribute two peak sun hours while a short clear one contributes six.

Most of the United States sits between three and six peak sun hours a year round, and the figure varies by more than a factor of two between the north and the south west.

A panel's rating is measured under laboratory conditions. Four hundred watts means 400 watts at a cell temperature of 25 degrees and full perpendicular sun, which is not what a roof gives you.

Real output is lower, and that gap is the system loss. Inverter conversion, cable resistance, dust, shading and the fact that hot panels produce less all come off the top, typically around 14 percent.

So the daily output of one panel is its kilowatt rating times the peak sun hours times one minus the losses. For a 400 watt panel at 4.5 hours and 14 percent loss that is 1.548 kWh a day.

Divide your daily use by that and round up. Thirty divided by 1.548 is 19.38, so 20 panels, because you cannot buy a fraction of one and because the array should cover the bad months too.

The system size follows directly: 20 panels of 400 watts is 8 kilowatts, which is the number installers quote and the number rebates are usually based on.

Roof area is the next constraint. A 400 watt panel covers roughly 21.5 square feet, so 20 of them want about 430 square feet, before setbacks and walkways are taken into account.

Roof orientation and tilt move the peak sun hours figure. A south facing roof at the right tilt gets the full value, while an east or west roof typically delivers ten to twenty percent less.

Coverage above 100 percent is normal and intentional. Because losses are already inside the calculation, a system that reports 103.2 percent of annual usage is sized to cover a typical year, not a perfect one.

Self consumption matters more than total production. A grid tied home exports its midday surplus, so what you save depends on when you use power as much as how much you generate.

Batteries change the arithmetic. Storage lets the surplus be used after dark, which raises self consumption but does not by itself reduce the number of panels needed.

Size for the loads you expect, not only the ones you have. An electric car adds roughly 3000 kWh a year and a heat pump can add more, so building in headroom now is far cheaper than adding panels later.